It has become clear in just the past two years that many mature cell lineages in digestive organs can dedifferentiate and re-enter the cell cycle to repair tissue damage. We and others have shown that post-mitotic cells can re-induce progenitor cell markers, re-enter the cell cycle, and can even serve as progenitors for other lineages. The dedifferentiation process carries a risk, however, as re-expressing progenitor cell genes is a form of cellular metaplasia. In states of chronic injury, like H. pylori infection in huan stomach, metaplasia continues indefinitely and increases risk for progression to cancer. Overall, cellular dedifferentiation presents a new paradigm for understanding tissue repair, stemness, and cancer initiation. Our overarching goal is to understand the cellular and molecular mechanisms governing dedifferentiation. Here, we will focus on the earliest stages of dedifferentiation in gastric digestive-enzyme secreting zymogenic chief cells (ZCs). Our preliminary and published data will show that the first stage is engulfment of secretory granules by lysosomes and that lysosomal turnover of the digestive enzyme secretory granules (secretory apparatus downscaling) must occur before progression to the next stages: metaplastic re- expression of progenitor cell genes and cell cycle re-entry. We will show the earliest molecular change in dedifferentiating ZCs is dramatically decreased expression of the bHLH transcription factor MIST1 (BHLHA15) and that deletion of Mist1 even in healthy ZCs is sufficient to induce lysosomal turnover of secretory granules. Finally, we will show that the only known transcriptional target of MIST1 that traffics lysosomes and is hence the most likely reason for the lysosomal attack on secretory granules when MIST1 is lost during dedifferentiation is the small GTPase, RAB26. Our aims are to: 1) determine the requirement/sufficiency for MIST1 in metaplasia downscaling; 2) determine if RAB26 affects flux through autophagy/mTOR signaling and whether it modulates granule destruction by lysosomes and/or autolysosomes; 3) a) determine if autophagic machinery is required for downscaling by inducing metaplasia following deletion in mature ZCs of the key autophagy genes Atg5 and Atg7 and b) to quantify autophagy/lysosome interaction with secretory granules in a database of human specimens where there are foci of ZCs undergoing dedifferentiation/metaplasia. Together the experiments will uncover for the first time the molecular mechanisms underlying the newly described cellular process of dedifferentiation and will have impact on human health because we will need to understand how metaplastic precursor lesions form from dedifferentiation of mature cells to be able to understand how to revert them and decrease risk for patients to progress to cancer.